1. Field of the Invention
The present invention relates to a device for mechanically connecting and assisting vertebrae with respect to one another, comprising at least one securing device presenting a ball joint extending from a vertebra and a compressible body maintained along the longitudinal axis of said securing device, said ball joint and said compressible element being housed between two end bodies of said securing device.
2. Brief Description of the Related Art
This type of device for mechanically connecting and assisting vertebrae with respect to one another acts outside the vertebral discs in order to compensate for defects of the spine, this making it possible to control and/or modify the movements of flexion and of traction between two vertebrae.
EP-A-0 611 554 discloses an intervertebral shock-absorber constituted by a cylindrical chamber receiving a ball joint extending from a vertebra, a piston capable of sliding in this cylindrical chamber as well as a compressible body. This cylindrical chamber is closed, at its end opposite the compressible body, by a screwed discoidal part. The volume of the elastic body, at rest, is less than that of the volume determined by the position of the piston with respect to the bottom of the cylindrical chamber.
This device allows a good intervertebral stabilization thanks to the presence of the compressible body which progressively resists the advance of the piston in exponential manner The connection between two devices of this type is effected by an assembly ring cooperating with tenons made on the corresponding ends of the cylindrical chambers.
In order to allow a relative movement in rotation between two adjacent securing devices, in the three dimensions in space, it is necessary that this assembly ring be made of a material allowing torsions in these three directions. This assembly member must therefore present a sufficient rigidity to connect two adjacent securing devices, as well as a considerable suppleness enabling it to twist in three directions. This compromise renders the design of such a ring very delicate, and its cost high.
In order to overcome these various drawbacks, the present invention aims at producing a device for mechanically connecting and assisting vertebrae, in which the securing devices may move mutually in rotation in the three directions in space, and which has a simple and reliable structure.